I. Field of the Invention
This invention relates to linings used for aluminum reduction cells.
II. DESCRIPTION OF THE PRIOR ART
Conventional aluminum reduction cells form a bath for a cryolite-based electrolyte containing dissolved alumina. Carbon anodes dip into the bath from above and are progressively consumed. The cell floor and walls are made up of carbon blocks bonded together with carbonaceous cement, or may be formed by using a rammed mixture of carbonaceous material and pitch. As electrolysis proceeds, a layer of molten product aluminum is built up on the carbon floor of the bath, from where it is tapped from time to time. The layer or "pad" of molten metal constitutes, together with the carbon floor, the cathode of the cell.
There are several disadvantages to the use of carbon linings for such cells. Firstly, the carbonaceous products used as carbon block cements or ramming mixes pose environmental hazards because of the fumes which are generated when such products are heated. Secondly, carbon linings are attacked by molten cryolite electrolytes and care has to be taken to protect such linings. Thirdly, strong magnetic forces generated by the high current levels associated with such cells give rise to magnetohydrodynamic effects which may cause instability of the molten metal pad.
Attempts have been made to find effective alternatives to carbon linings, for example as disclosed in our U.S. Pat. No. 4,647,357 to Dewing issued on Mar. 3, 1987 and in our European patent application Ser. No. 86306584.3 filed on Aug. 26, 1986 (and published on Mar. 25, 1987 under Publication No. 0215590). While the materials disclosed in these references are extremely effective, their use requires a modification of the normal cell design because current must be collected from the molten metal pad by high temperature-resistant refractory hard metal current collectors which extend upwardly through the non-conductive layer. Consequently, the new materials cannot conveniently be used to replace the bottom linings of existing conventional aluminum reduction cells.